While you buy miners, they buy bitcoins, the bitcoin price raise more fast than the bitcoins you can mine with the miners.

When you send them 25 000 USD for a ASIC, they buy bitcoins, by the time you get your machine, their bitcoins double in price, they make profits. You instead will have to wait much longer to maybe break even.

It's more profitable to sell the machines and buy bitcoins for them than mine for bitcoins.